The Integumentary System: Structure, Function, and Clinical Aspects

The Integumentary System

Overview

The integumentary system is the body's largest organ system, serving as a protective barrier and playing vital roles in sensation, thermoregulation, and metabolic processes. It consists of the skin, sweat glands, oil glands, hair, nails, and subcutaneous tissue.

• Skin: The primary organ, composed of two main layers (epidermis and dermis) and an underlying subcutaneous layer.

• Appendages: Includes hair, nails, and cutaneous glands (sweat and sebaceous glands).

5.1 Skin Structure

Layers of the Skin

The skin is composed of two main layers and an associated subcutaneous layer:

• Epidermis: Outermost, avascular, composed of keratinized stratified squamous epithelium.

• Dermis: Deeper, vascular, made of dense connective tissue.

• Subcutaneous Tissue (Hypodermis): Not part of the skin proper, but anchors skin to underlying structures, provides insulation and shock absorption.

5.2 The Epidermis

Cell Types in the Epidermis

• Keratinocytes: Produce keratin, the protein that gives skin its protective properties; most abundant cell type.

• Melanocytes: Produce melanin pigment, which protects against UV radiation.

• Dendritic (Langerhans) Cells: Immune cells that patrol the epidermis and activate immune responses.

• Tactile (Merkel) Cells: Sensory receptors for touch, located at the epidermal-dermal junction.

Layers (Strata) of the Epidermis

The epidermis is organized into distinct layers:

• Stratum Basale: Deepest, single row of mitotically active stem cells; contains melanocytes.

• Stratum Spinosum: Several layers thick, contains pre-keratin filaments and abundant dendritic cells.

• Stratum Granulosum: Cells flatten, organelles disintegrate, keratinization begins, and glycolipids are released for waterproofing.

• Stratum Lucidum: Thin, clear layer found only in thick skin (palms, soles).

• Stratum Corneum: Outermost, 20–30 layers of dead, keratinized cells providing protection.

5.3 The Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer housing blood vessels, nerves, hair follicles, and glands. It is divided into two layers:

• Papillary Dermis: Superficial, areolar connective tissue with dermal papillae that contain capillaries, nerve endings, and touch receptors.

• Reticular Dermis: Deep, dense irregular connective tissue with collagen and elastic fibers, providing strength and elasticity.

Dermal Modifications

• Friction Ridges: Form fingerprints, enhance grip and tactile sensation.

• Cleavage (Tension) Lines: Orientation of collagen fibers; important for surgical incisions.

• Flexure Lines: Dermal folds at joints, visible on palms and fingers.

5.4 Skin Color

Pigments Determining Skin Color

• Melanin: Only pigment produced in the skin; protects DNA from UV damage. Varies in amount and type (reddish-yellow to brown-black).

• Carotene: Yellow to orange pigment, most visible in palms and soles; can be converted to vitamin A.

• Hemoglobin: Pinkish hue in fair skin due to low melanin and visible blood flow.

Alterations in skin color can indicate disease (e.g., cyanosis, jaundice, pallor, erythema, bruising).

5.5 Hair

Structure and Function

• Hair (Pili): Dead, keratinized cells produced by hair follicles; functions in protection, sensation, and filtration.

• Regions: Root (in scalp, keratinization ongoing) and shaft (above scalp, keratinization complete).

• Hair Shaft Layers: Medulla (core), cortex (middle), cuticle (outermost).

Hair Follicle Structure

• Follicle Wall: Peripheral connective tissue sheath, glassy membrane, epithelial root sheath.

• Hair Bulb: Expanded base with hair matrix (growth zone) and papilla (nutrient supply).

• Arrector Pili: Smooth muscle causing "goose bumps."

Types and Growth of Hair

• Vellus Hair: Fine, pale body hair of children and adult females.

• Terminal Hair: Coarse, long hair (scalp, eyebrows, axillary, pubic regions, face/neck in males).

• Growth cycles alternate between active and resting phases; affected by nutrition and hormones.

5.6 Nails

Structure and Function

• Nails: Scale-like modifications of the epidermis containing hard keratin; protect distal phalanges.

• Parts: Root, nail plate (body), free edge, nail bed, nail matrix (growth zone), nail folds, cuticle (eponychium), hyponychium.

Abnormal nail appearance can indicate systemic disease (e.g., spoon nails in iron deficiency, Beau's lines in severe illness).

5.7 Sweat and Sebaceous Glands

Sweat (Sudoriferous) Glands

• Eccrine (Merocrine) Glands: Most numerous, abundant on palms, soles, forehead; function in thermoregulation; secrete hypotonic sweat (99% water, salts, wastes).

• Apocrine Glands: Axillary and anogenital areas; secrete viscous, protein-rich sweat into hair follicles; function begins at puberty; may act as scent glands.

• Modified Apocrine Glands: Ceruminous (earwax) and mammary (milk) glands.

Sebaceous (Oil) Glands

• Widely distributed except on palms and soles; secrete sebum (oily, bactericidal, softens hair and skin).

• Most develop from hair follicles; holocrine secretion method.

Summary Table: Cutaneous Glands

	Eccrine Sweat Glands	Apocrine Sweat Glands	Sebaceous Glands
Function	Temperature control, some antibacterial properties	May act as sexual scent glands	Lubricate skin and hair, prevent water loss, antibacterial
Type of Secretion	Hypotonic filtrate of blood	Filtrate of blood with proteins and fatty substances	Sebum (oily secretion)
Method of Secretion	Merocrine (exocytosis)	Merocrine (exocytosis)	Holocrine
Secretion Exit	Skin surface	Upper part of hair follicle	Upper part of hair follicle or skin surface
Location	Everywhere, especially palms, soles, forehead	Axillary and anogenital regions	Everywhere except palms and soles

5.8 Functions of Skin

Main Functions

• Protection: Chemical (acid mantle, melanin), physical (keratinized cells), and biological (immune cells) barriers.

• Body Temperature Regulation: Sweat production and blood vessel dilation/constriction.

• Cutaneous Sensations: Touch, pressure, pain, and temperature receptors.

• Metabolic Functions: Vitamin D synthesis, activation of hormones, collagenase production.

• Blood Reservoir: Holds up to 5% of blood volume.

• Excretion: Removal of nitrogenous wastes, salt, and water via sweat.

5.9 Skin Cancer and Burns

Skin Cancer

• Basal Cell Carcinoma: Most common, least malignant; arises from stratum basale; high cure rate with excision.

• Squamous Cell Carcinoma: Arises from keratinocytes of stratum spinosum; can metastasize; good prognosis if treated early.

• Melanoma: Cancer of melanocytes; most dangerous due to high metastatic potential and resistance to therapy.

Burns

• First-degree: Epidermal damage only; redness, pain, swelling.

• Second-degree: Epidermal and upper dermal damage; blisters form.

• Third-degree: Full-thickness burn; skin appears gray-white, cherry red, or blackened; not painful (nerve endings destroyed); requires grafting.

Severity is estimated using the "rule of nines" to calculate fluid loss and extent of burns.

Developmental Aspects of the Integumentary System

Embryonic to Adult Development

• Skin develops from ectoderm (epidermis) and mesoderm (dermis, subcutaneous tissue).

• At birth, skin is covered with vernix caseosa (protective substance) and lanugo (fine hair).

• With age, skin thickens, glands become more active, and hair patterns change.

• Aging leads to thinning, dryness, decreased elasticity, and increased risk of cancer.